Microelectrodes will be used for the recording of transmembrane potential, voltage-clamp and iontophoretic intracellular injection of drugs to study the electrical activity of normal and diseased atrial fibers from normal and diseased human and canine atria, the electrical activity of normal and abnormal canine cardiac Purkinje fibers and fibers of the rabbit atrioventricular node in order to provide an improved understanding of the electrophysiological mechanisms responsible for abnormalities of cardiac rhythm and conduction and their modification by pharmacologic agents. Other studies will be conducted on the in situ canine heart using standard electrophysiological and pharmacologic techniques, to relate findings for isolated cardiac tissues to the activity of the intact heart. The primary goal of these studies is to provide a basis for the identification of mechanisms responsible for clinically important arrhythmias and conduction disturbances so that a rational framework for therapeutic intervention will be possible.